Carb spacer effect on a piston ported cylinder?

[Iron.and.bark]

Just curious, but I was wondering if someone could enlighten me on what the effect is if you either reduce or increase the length of the existing spacer between cylinder and carb. I may not be using the correct term, I am reffering to the intake track area that is either a solid block or ribbed hose etc.

I mean for piston ported cylinders, not the reed blocks. Know how it effects them

Cheers

 

[angelo c]

Intake manifold ?
Carb Venturi ?

 

[Iron.and.bark]

I guess maybe intake runner may be another term. Played with them on cars (13b rotary) and lengthening the intake track post carb gave the illusion of more power as it shifted the torque band lower.

I am just curious on the behavior, will iy be the same for a piston ported 2 stroke?

 

[Terry Syd]

Same thing, you're just tuning the resonance of the intake tract. You can increase the length or volume or both in order to change when the positive wave hits the closing intake port and packs some extra mixture into the crankcase.

It would be a neat addition to a saw to have a 'boost bottle' on a valve to change the powerband. You could tune the intake for a higher RPM when working smaller wood and then re-tune it lower for when you were bucking bigger wood.

Chainsaws are pretty basic and need to be to be lightweight. The scavenging efficiency of a piston port chainsaw is around 75%. Compare that to a high performance two-stroke with around 95% scavenging efficiency and you can to begin to understand how a crappy open-port cylinder can still work relatively well on a chainsaw.

 

[CR888]

'Intake tract resonance tuning' is the term. Where it becomes a little difficult with a saw is sufficient room/space to properly do it. Other 2T applications use it with good effect. I'd guess you would need 6"-9" of intake tract to dial it in. Like resonant pulses are used for expansion pipes intake tract tuning can benefit too. Its really cool stuff making use of what is already there as a bi product of 2T combistion. Terrysyd likes the subject and has thought of adding a boost bottle to the intake.

 

[Chainsaw Jim]

Same thing, you're just tuning the resonance of the intake tract. You can increase the length or volume or both in order to change when the positive wave hits the closing intake port and packs some extra mixture into the crankcase.

It would be a neat addition to a saw to have a 'boost bottle' on a valve to change the powerband. You could tune the intake for a higher RPM when working smaller wood and then re-tune it lower for when you were bucking bigger wood.

Chainsaws are pretty basic and need to be to be lightweight. The scavenging efficiency of a piston port chainsaw is around 75%. Compare that to a high performance two-stroke with around 95% scavenging efficiency and you can to begin to understand how a crappy open-port cylinder can still work relatively well on a chainsaw.

Why is open port "crappy"?

 

[rogue60]

My understanding a boost bottle basically tricks the engine into thinking it has a longer intake/more volume. Simple enough to try on a saw so whos going to try it? works on piston port or reed valve engines, here's a pic of of a DIY one for about 50cc (not mine) they say they work? all the calculations are out there for this type mod on a 2T, the bottle cc is important as is the length of pipe all matched to engine cc etc.
I owned a few Yamaha's over the years with boost bottles YEIS (Yamaha Energy Induction System) but never did try taking one off to see what difference if any there was in performance.

 

[Terry Syd]

Why is open port "crappy"?

On a chainsaw they can work, but you won't find them on high performance two-strokes. The open port doesn't have the ability of a closed port jug to direct the transfer flows to fine tune the loop-scavenging, plus the larger bridge between the ports adds extra turbulence/mixing to the flow. However, with the lower scavenging efficiency of the chainsaw being around 75%, you are looking at 25% of the cylinder charging being spent exhaust gases. So, there is going to be considerable mixing of the charge in any case and the open port jug can work with that level of scavenging efficiency.

Maybe I should have called the open port an 'industrial' design.

 

[Terry Syd]

all the calculations are out there for this type mod on a 2T,

I've tuned intakes and the calculations will get you close, but you have to spend some time tweaking things to get it where you want it. There is also a primary wave and then subsequent waves to deal with. If you get it right you can use a secondary wave at a different part of the powerband.

The hollow handle on a chainsaw could add volume to the intake tract and act as a 'boost bottle' for the saw. Using the existing handle would save weight and simplify the construction.

 

[RI Chevy]

I was kind of wondering the same thing. Like a tuned port injection intake manifold on a car. An intake with longer runners to help with torque. It could possibly be done with the green weanie aluminum stack type setup. Just a little bit longer on the aluminum stack mount. I wonder if it would make any difference in a 2 stroke chainsaw setup?

 

[CR888]

I was going to buy a boost bottle used for mopeds and give it a go. I chickened out but they were around $35 on eBay with hose and connections for the Chinese 50cc/65cc/80cc motors. AM performance parts for mopeds seem to be really easy to buy online, stuff for saws is a bit more tricky.

 

[Chainsaw Jim]

Isn't that one of the differences between the 076av and the 076 super? A high rise stack?

 

[paragonbuilder]

I was kind of wondering the same thing. Like a tuned port injection intake manifold on a car. An intake with longer runners to help with torque. It could possibly be done with the green weanie aluminum stack type setup. Just a little bit longer on the aluminum stack mount. I wonder if it would make any difference in a 2 stroke chainsaw setup?

Doesn't it need to be post carburetor?

 

[RI Chevy]

Not sure Dan. Long runners usually produce slightly more torque and HP in Autos. I am very confused with 2 stroke stuff.
Heck, its worth a shot to see. But then again, it may hurt it as well. ???

 

[rogue60]

Not sure Dan. Long runners usually produce slightly more torque and HP in Autos. I am very confused with 2 stroke stuff.
Heck, its worth a shot to see. But then again, it may hurt it as well. ???

Some good reading it does cover some on intake lengths and such ,but comes with a warning may induce a headache lol
http://www.amrca.com/tech/tuners.pdf

 

[MustangMike]

Cars are way different, rigid intakes, multiple cylinders. Would be harder to tune properly on a saw.

 

[Iron.and.bark]

Some good reading it does cover some on intake lengths and such ,but comes with a warning may induce a headache lol
http://www.amrca.com/tech/tuners.pdf

It's christmas day. Going to have a headache anyway

 

[Terry Syd]

Here's a better document to get a headache with - checkout 'loopsaw' in the document. It appears to be a Stihl chainsaw that they used.

http://dragonfly75.com/motorbike/2StrokeDesign.pdf

Don't worry about the mathematics, that is only to prove up the theories and give guidance to future design applications. The concepts are in the text along with various graphs to illustrate the concepts.

There are all sorts of ways to do these resonance tuning mods. One of the early motorcycles, a slow turning 4-stroke, developed an exhaust pipe that worked well for the engine. Unfortunately, it was something like 8 feet long. How do you get an 8' exhaust pipe on a bike?

What they did is cut the pipe off at the end of the rear tire and then added an 'interference' pipe to the side of the pipe. The interference pipe was closed off and ran in parallel with the main pipe. It 'fooled' the engine in thinking it was an 8' pipe. I did a similar mod to a bike where instead of a 'boost bottle' I used a long section of plastic hose. I kept trimming the hose until I got the right volume, then put in the end plug. The hose ran up and under my rear fender and was about 3' long.

 

[wcorey]

Doesn't it need to be post carburetor?

AFAIK it doesn't matter, can also be a long velocity stack.

The air cooled/carburated Ducati twins I used to work on had relatively long intakes, there was a short intake conversion the race guys would use to get a small power advantage up top but mostly the street guys didn't like them because it narrowed the power band very noticeably.
They also employed 'spaghetti' exhausts that twisted, turned and looped all over both to gain length and even up the length between front/rear cylinders.

The fuel inj/water cooled models were sensitive to airbox volume, lack of volume was causing significant 'dips' in/along the power curve, I remember a bad one around 7k.
We'd cut out the whole floor of the airbox and bring it down below/around the throttle bodies with a carbon fiber tub/extention. I still have a bunch of fiberglass molds and fixtures I made for those.

 

[CTYank]

Cars are way different, rigid intakes, multiple cylinders. Would be harder to tune properly on a saw.

Not necessarily. Take a look at the induction "forest" on a big-block light-alloy engine of a McLaren Can-Am car of days past. Point is, each cyl has its private intake tract. Chrysler had some interesting "cross-fire" manifolds back when, again with each cyl having its own long induction tract.
Just too bad that piston-ported 2T engines have to have symmetrical port timing about TDC/BDC. Rotary valves would be cool, just bulky.